A conventional listening device such as a hearing aid includes, among other things, a microphone and a receiver. The microphone receives sound waves and converts the sound waves to an audio signal. The audio signal is then processed (e.g., amplified) and provided to the receiver. The receiver converts the processed audio signal into an acoustic signal and subsequently broadcasts the acoustic signal to the user.
The microphone generally has a rigid, electrically charged backplate and a moveable metallic diaphragm. The diaphragm divides the inner volume of the microphone into a front volume and a rear volume. Sound waves enter the microphone via a sound inlet and pass into the front volume. The air vibrations created by the entering sound waves cause the metallic diaphragm to move, thereby inducing an electric signal in the electrically charged backplate corresponding to the sound waves. The electric signal is then processed by audio processing circuitry connected to the charged backplate and converted into an audio signal.
For certain applications, including hearing aids and other listening devices, it is desirable to dampen the resonance frequency of the microphone system. One way to dampen the frequency response is to increase the inertance presented to the sound waves entering the microphone by placing an obstruction near the sound inlet in the front volume. Common types of obstructions include a damping screen made of a grid-like mesh material placed over the sound inlet, a shaped embossment or structure formed or placed inside the housing of the microphone near the sound inlet, and the like.
A damping screen, however, can become clogged as debris and foreign material accumulate on its surface. As the dampening screen becomes increasingly clogged, the microphone's frequency response may depart from the specification. A shaped structure can also become less effective as debris accumulates, since the shaped structure depends on its shape to create the desired dampening effect. If the accumulated debris alters the shape of the shaped structure, the microphone's frequency response will be altered. In both of the above cases, the accumulation of debris, such as dust, hairspray, pollen, and other particles, may adversely affect the frequency response of the microphone and may even cause it to malfunction.
Unlike the front volume, the rear volume is typically sealed off and largely impervious to debris. Therefore, some microphones place the damping mechanism in the rear volume to avoid debris accumulation. These microphones use a damping frame between the diaphragm and the backplate to dampen the frequency response. The damping frame has inner slits cut into its opposing edges that, together with the backplate, define apertures through which air may escape from the area between the diaphragm and the backplate to the rest of the rear volume. The escaping air results in a damping of the frequency response of the microphone. An example of such a microphone may be found in commonly-owned U.S. Published Application No. 20030063768 to Cornelius et al., which is incorporated herein by reference in its entirety.
The dimensions of the inner slits in the above microphones have to be very precise in order to achieve the desired level of escaping air for damping purposes. Also, the damping frame is normally made of a stiff or rigid material, usually plastic or Kapton®. Moreover, a hole is sometimes punched through the backplate to facilitate handling during assembly of the microphone. This hole has to be subsequently filled (e.g., with adhesive or similar material) in order to prevent air from escaping through the hole. Accordingly, what is needed is an improved way to control the frequency response of the microphone.